how to monitor hard drive disks activity ?

My system has no visible means - no "busy light" to indicate hard drive disk activity.
But I can hear them grinding.

My cheap USB sticks lights-up when activated.

Is there a generic Linux application to tell me when my HDD disks are in use.
Such as "gparted" "scanning for devices" or mdadm / RAID updating ?
But generic - triggered by ANY disk activity.

Cheers